Fix conflicts between DH check flags and FFC check flags

Fix conflicts between DH check flags and FFC check flags

There are comments in include/openssl/dh.h and include/internal/ffc.h
that they must be aligned with each other, and yet, clashes have been
introduced.

The simplest fix is to move the offending FFC flags out of the way, as they
are indeed internal and shouldn't affect any public interface, apart from
those that are aligned with the DH flags, which are public.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22453)